Fixes #3100 - Add constructor overload to SliderOption<T>

[tznind]

Fixes #3100- Adds new constructor to SliderOption<T> for populating values (Legend etc).

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tznind]

@tig when you get a chance can you please review this and also #3103 as this will unblock my work on Slider support in TerminalGuiDesigner

[tznind]

Thanks <3